This invention relates to a voltage converter system for changing a voltage applied across a capacitive load such as a beam penetration type multi-color cathode ray tube at a high speed with a high efficiency.
Beam penetration type multi-color cathode ray tubes are color cathode ray tubes of the type emitting light in a color which is variable between red and green and is controlled by varying the energy of an electron beam involved through a change in voltage applied between the anode and cathode electrodes thereof. This type of color cathode ray tube is presently in the limelight because it is able to effect a colored graphic display. However, in order to display a multiplicity of data, as in the color display used with radars, it is necessary to switch the high voltage across the capacitive load within a time interval which is not longer than about 15 microseconds. Furthermore with such cathode ray tubes being shielded electromagnetically, the anode side thereof has such an increased capacitance to ground that it is more difficult to change the voltage thereon at a high speed.
For these reasons, it has been considered that in general beam penetration type multi-color cathode ray tubes, the time interval for changing the voltage or color has a limit ranging from several tens to several hundreds microseconds.
There have been already proposed various types of voltage converter systems for changing the voltage across a capacitive load. One of the proposed voltage converter systems comprises a fixed voltage source such as a bias power source, connected to one end of the capacitive load and a pair of identical variable voltage sources connected to the other end of the capacitive load and the fixed voltage source respectively. Each of the variable voltage sources has a capacitor for sharing and holding a constant voltage, a pair of series combinations of a polarity inverting switch and a semiconductor diode connected across the capacitor through a reactor to invert the constant voltage from the positive to negative polarity and vice versa respectively, and a low impedance DC source connected to each series combination of the inverting switch and diode through a compensating switch to compensate for a loss in electrical energy due to the inversion of the voltage's polarity. Each of the DC sources is connected to the junction of the capacitor and the reactor through a semiconductor diode to form a bypass path for a current through the capacitive load.
When a load composed of the capacitive load and the capacitor is capacitive, the voltage across each of the variable voltage sources is skillfully inverted in polarity through the utilization of the current exchange between the reactor and the composite load. Therefore, the current flowing through an associated one of the inverting switches becomes equal to zero when the capacitive load and the capacitor have been completely charged while at the same time the voltage across the capacitor has been inverted in polarity. Under these circumstances, a thyristor switch may be substituted for each of the inverting switches 40, 44, 40' and 44' (see FIG. 5) to open an associated circuit when the abovementioned current becomes equal to zero through the self commutation.
However, with composite loads having large resistive portions, it is difficult to effect the inversion of the applied voltage and the commutation smoothly. Therefore, the use of only thyristor switches leaves problems with respect to the standpoint of stable operation. For this reason, transistorized switches have inevitably been employed to handle the load of the type referred to with high inverted voltages, and it has been necessary to connect the transistors of such switches in a multi-stage cascade manner. Consequently, problems occur in that the resulting circuit configuration including bias circuits is complicated as a whole. The object of the present invention is to provide a new and improved voltage converter system for changing the polarity of a voltage applied across a capacitive load at a high speed.
It is another object of the present invention to provide a new and improved voltage converter system having a simplified circuit configuration.
It is still another object of the present invention to provide a new and improved voltage converter system which may be effectively used with a beam penetration type multi-color cathode ray tube.